A gene modifies the electrophoretic mobility of primary gene products of two malate dehydrogenase loci (Mdh2 and Mdh3). Haploid endosperm and corresponding embryos of open-pollinated single trees and dormant buds were analysed in Norway spruce (Picea abies). Whenever the modifying gene was detected, both isoenzymes encoded by Mdh2 and Mdh3, respectively were affected. Allozyme Mdh2-100 is shifted to Rm 105, allozymes Mdh3-100 and Mdh3-122 are shifted to Rm 112 and Rm 136, respectively and mobility of Mdh3-140 is changed is changed to Rm 152. Electrophoresis of haploid tissue of heterozygous trees at one structural locus and the modifier gives rise to four distinguishable electrophoretic phenotypes. This provides evidence that the modified MDH isozymes are no allelic products of the MDH structural locus. Allelic variation at one structural locus results at most in two phenotypes. Additional phenotypes have to stem from post-translational modification by the action of another gene. Tissue of buds originating from heterozygous trees at the modifying gene have wild-type electrophoretic phenotype. In diploid tissue derived from buds and embryos, the modified electrophoretic phenotype is only expressed if the tree is homozygous for the modifying allele. Therefor, it can be concluded that the modifying allele is recessive. Linkage analysis are under investigation, using segregation data of haploid endosperm.
